A solid model describes the mapped organ in the form of 3D-image data in which information on one of its material characteristics is given for individual volume elements (voxels) of the organ. An individual volume element may, for example, represent a rectangular-shaped section of the organ. The section may correspond to a volume of e.g. one cubic millimeter. The information for each volume element is obtained from 2D image data of the X-ray images which can be obtained with a 2D X-ray detector of the C-arm X-ray unit. The trajectory of the X-ray beams back to the X-ray focus of the X-ray source of the C-arm X-ray unit is reproduced by each pixel (picture element) of the X-ray detector. A possible algorithm for this is the sufficiently well-known backprojection, in particular the filtered backprojection, for example, of the Feldkamp type.
A C-arm X-ray unit is now characterized in that the X-ray source on the one hand and the 2D X-ray detector on the other hand are arranged at the ends of a C-shaped, rotatable metal arm. By rotating the C-arm around the body of a patient, the X-ray images which are necessary for the calculation of the voxel-based solid model of an organ of the body can be generated from various projection directions.
However, for this calculation a special feature of C-arm X-ray equipment must be noted compared with an X-ray scanner with a gantry. The C-arm displays significantly less rigidity than a gantry. The C-arm is deformed as a result of the net weight of the C-arm and the weight of the X-ray source and of the X-ray detector as well as by centrifugal forces during the C-arm rotation. This results in a different geometry of the C-arm X-ray unit for every X-ray image depending on the orientation of the C-arm and its rotational speed. A different geometry must therefore be taken as the basis for the calculation of the trajectories of the X-rays for each X-ray image.
Due to the complexity of the influences, reliance on calibration is necessary here. This is performed by means of a calibration body which can, for example, be a water-filled ball. Several X-ray images are generated by the calibration body from different projection directions. In this connection, the C-arm is moved at a predefined rotational speed around the calibration body at a predefined angle of rotation to be traversed. The X-ray images are then generated from respective predefined projection angles. The guidelines for the rotational speed, the total angle of rotation traversed and the individual projection angles for the images together define an imaging protocol for the C-arm X-ray unit.
A solid model of the calibration body is then calculated from the X-ray images. Based on the shape of the solid model of the ball, projection data can then be ascertained for the individual X-ray images, by means of which a distortion-free image of the calibration body is obtained in the solid model. Such projection data can, for example, be provided in the form of a projection matrix by means of which the 2D image data of the X-ray images is processed within the framework of a backprojection. Generally speaking, the projection data describes the trajectories of the X-rays. A projection data set for an X-ray image thus determines how the 2D image data of the X-ray image is to be introduced into the solid model.
If a solid model is now subsequently generated from a body of a patient by means of the same C-arm X-ray unit and in this connection the same imaging protocol is traversed to obtain the X-ray images as was used for calibration by means of the calibration body, then the projection data can also be re-used. In this connection it is therefore customary to save several imaging protocols for a C-arm X-ray unit and to have the projection data sets ready for each imaging protocol.
However, in connection with the generation of a solid model of a heart, a problem exists in that the heart moves periodically. A (static) 3D solid model can only represent a particular movement phase of the heart. A common specification of such a movement phase, or phase for short, is the percentage of the RR interval, in other words, for example, 70% RR. The RR interval indicates the period of time between two contractions of the ventricles of the heart. As is known, in an electrocardiogram the start of the contraction of the ventricle appears as a so-called R-wave. Correspondingly, the time lag between two R-waves corresponds to the RR interval.
With a C-arm X-ray unit it is simply not possible to now monitor an electrocardiogram signal and to generate an X-ray image whenever the heart is in the desired phase of its periodic pumping movement, in other words, for example, at 70% RR. Such EKG triggering by an EKG trigger in particular means that the trigger times for the X-ray images are dependent on the heartbeat. For this reason, however, a prepared protocol with fixed predefined recording times and projection angles cannot be executed.
A known solution to this problem is to prepare a protocol by means of which during a C-arm rotation X-ray images are generated in close temporal succession. On the basis of this imaging protocol, projection data sets for generating a solid model are ascertained via calibration. By means of the imaging protocol, X-ray images of the heart are then obtained which show the heart in different phases. In the meantime, an electrocardiogram signal is recorded at the same time. Subsequently, on the basis of the EKG signal (EKG—electrocardiogram), those X-ray images are then identified and selected in which the heart is shown in the desired phase. The other X-ray images are discarded. A disadvantage of this solution is that a wide variety of X-ray images is required, which is reflected in an undesirably high dose of radiation for the patient.